The overall aim of this project is to define at both the cellular and molecular level, the signalling events involved in IgA heavy chain (CH) class switching. Studies during this period focused on the regulation of isotype switching in the antigen (SRBC)-reactive B cell line CH12.LX as well as on normal, purified IgM(+) B cells. CH12.LX cells comprise a stable population of B cells bearing sIgM (98-99%) as well as a small population of cells bearing sIgA (1- 2%). Studies with recombinant lymphokines disclosed that rIL-4 induces an increase in the percentage of sIgA(+) cells (to 6-15%) with minimal effects on IgA secretion, whereas IL-5 induced increased IgA secretion, but had no effect on sIgA expression. The addition of both lymphokines induced both increased IgA expression and IgA secretion. No effect on mIgA expression or IgA secretion was seen with other lymphokines including IL-l, IL-2, IL-3, IL-6, GM-GSF and IFN-gamma. In further studies, it was shown that IL-5 causes IgA secretion by sIgA(+) but not sIgA(-) CH12.LX cells indicating that IL-5 is acting as a post-isotype switch differentiation factor. Thus, these studies indicate that IL-4 and IL-5 act in a sequential fashion to induce IgA expression and secretion in the CH12.LX system: IL-4 inducing differentiation of sIgM(+) cells to sIgA(+) cells and IgA enhances the IgA secretion of the resulting sIgA(+) cells. Complementary studies with normal B cells (derived from Peyer's patches) utilized cells separated into sIgA(+) and sIgA(-) cells obtained by panning or sorting with by FACS. Here, LPS-driven sIgA(+) B cells were induced by rIL-5 to secrete increased amounts of IgA; in contrast, mIgA(-) cells manifested no such secretion. Thus, it could be shown that IL-5 acts on normal B cells as a postswitch, factor, promoting differentiation of IgA B cell into IgA secreting plasma cells.